The invention relates to an illumination system in a projection copier for the illumination and projection of an original onto a light-sensitive recording material, with an original platform to receive the original, with radiating sources which are partially surrounded by reflectors, which reflect the incident radiation in the direction of the original.
German Patent Specification No. 2,452,979 discloses such an illumination system in which two radiating sources are arranged on the longitudinal sides, symmetrical to the center line, of the illumination system and are partially surrounded by mirror reflectors, which reflect the incident radiation in the direction of further reflectors. There are two further radiating sources, partially surrounded by mirror reflectors, on the transverse sides of a mount, which mount surrounds an entrance opening of a bellows, on the longitudinal sides of which the previously mentioned radiating sources are provided with the associated mirror reflectors. The further reflectors are likewise designed as mirror reflectors with a flat, strip-shaped middle section, to which concavely curved sections adjoin on either side and extend over the depth of the illumination system. The mirror reflectors and the lower curved sections of the further reflectors are opposite one another, the further mirror reflectors being covered in the direction of the original holder in their entirety by a flange, which projects outward along the border of the mount. Covering the further mirror reflectors by a flange has the effect that, in illumination of the original, an imaging of the same on the recording material is achieved, while avoiding the silhouettes of the edge of the original and without reducing the luminous efficiency.
This known illumination system has no precautions for keeping the thermal radiation of the radiating sources away from the original. If particularly high-power radiating sources are used, as is the case with projection copiers for a large throughput of recording materials, such as for example printing plates, the development of heat within the illumination system is considerable and a corresponding heating-up of the original platform and of the respective original may occur, possibly resulting in undesired thermal stresses in the original holder, which generally has a glass plate, which could adversely affect the projection accuracy onto the recording material.
In U.S. Pat. No. 3,428,397, a copier is described in which symmetrical arrangements of light reflectors project the light onto an original. Each arrangement has a curved mirror reflector, which partially surrounds an elongated light source, and surface without an antireflection coating, diffusely distributing the light, and located between the light source and the original platform.
The curved mirror reflectors consist of two connected parts which have different curvatures from each other. Adjoining each of the mirror reflectors in the upward direction is a non-specular, diffusely reflecting wall which is inclined outward with respect to the longitudinal axis of the illumination system. Similarly, each curved reflector continues downward into a nonspecular, diffusely reflecting wall, which is likewise inclined with respect to the longitudinal axis of the illumination system and rests with its bottom end on the optical lens. The inclination of the lower wall with respect to the longitudinal axis is greater than that of the upper wall. The lower part of the mirror reflector reflects light onto the opposite upper wall, while the upper part of each mirror reflector reflects light onto the opposite lower wall. The light falling on the walls is reflected from them diffusely in all directions. Part of the light from the mirror reflectors is also directed directly onto the original platform. The latter is thus illuminated by light which is directly reflected by the mirror reflectors and by diffuse light which is radiated onto the original platform after several reflections at the diffusely reflecting walls. As well as the inclined, diffusely reflecting walls, this illumination system also has vertical side walls which are highly reflecting. The light rays falling onto these vertical walls are not deflected onto the lens after corresponding reflection as they are covered by corresponding parts of the mirror reflectors. A certain proportion of the diffusely reflected light which falls on the original platform is directly reflected in the direction of the lens. Nevertheless, it has been found that this mirror reflection makes up such a small proportion of the total reflection of the diffusely illuminated original on the original platform that an impairment of the copy quality by the occurence of streaks or punctiform imaging in the copy is negligible. In the case of this illumination system, although a direct illumination of the plane of the original by the light source is avoided by the upper parts of the mirror reflectors drawn towards the middle, an illumination of the borders of the plane of the original by light directly reflected by the mirror reflectors is not avoided. The physical distance of the light sources from the plane of the original is not equal to the depth of the illumination system. The diffusely reflecting walls surround the optical lens for the imaging of the original onto an image carrier. The reflecting elements arranged along one side of the illumination system, namely the upper wall, the mirror reflector and the lower wall, adjoin one another without interruption and are not opposite one another with their reflecting surfaces. The special design of the individual mirror reflector, which consists of two parts with different curvatures, makes manufacture both complex and expensive. Since the upper and lower walls are only diffusely and not specularly reflecting, the luminous efficiency for illumination of the original is small in comparison with the radiant power of the light sources.
U.S. Pat. No. 3,272,066 discloses an illumination system which has four lamps in the four corners of a chamber of the projection copier, which lie opposite to an original window. In the front of each lamp is fastened a reflection plate, which prevents the light of the lamp concerned from falling directly onto the original window and being reflected from there onto a lens optical system. The flat side walls of the chamber are of reflecting design and deflect the lamp light onto the original window. The four lamps of this illumination system interact with the upstream deflection plates to direct the light preferably into the corners of the original window. The deflection plates ahead of the lamps prevent only a direct radiation of the light rays into the center of the original window, but not into its corners as it is a declared aim of the known illumination system to increase the light intensity in the corners in relation to that in the center. The use of a deflection plate for each lamp makes for increased structural complexity.
German Offenlegungsschrift No. 2,160,439 describes an illumination system in which reflectors cast the light from light sources onto a translucent receiving plate for the original. In the case of this illumination system, no measures are taken to prevent a direct illumination of the original and a reflection of the incident light from the first reflectors onto the original in order to obtain a completely shadowless edge illumination of the original. To achieve this, additional light sources are provided instead which eliminate the shadows on the recording material created by the border areas of the original.
Swiss Patent Specification No. 489,818 relates to an illumination system with a reflector system, the individual part-reflectors of which form the wall parts of the equipment housing. In this case, the light source is arranged in a cross-section of the system which is laterally offset with respect to the original holder. Owing to the lateral arrangement of the light source, although there are hardly any problems in avoiding the direct illumination of the original by the light source, several reflectors are necessary to be able to direct the entire radiation emitted by the light source onto the exposure opening in order to obtain a satisfactory light efficiency. The superposition of three light components in this part-reflector system produces a greater emission angle in comparison with one or two reflectors, making possible the illumination of a larger original area since the entire original area, including its border regions, is adequately lit. For this purpose, both a deflection reflector and an echelon reflector deflect the incident radiation directly in the direction of the original, no reflection via a second reflector taking place. The structural complexity of this system is disproportionately great.
German Offenlegungsschrift No. 2,139,326 discloses an illumination system in which, for the illumination of the plane of the original, there are four light sources arranged in one plane and symmetrical to the original. These light sources illuminate, among other things, the original directly, so that the imaging of the shadows of the edges of the original on the copy cannot be excluded.
In the case of the known illumination systems, no measures are taken to keep away the infrared component, i.e., the heat rays of the radiation sources from the original and at the same time to obtain a completely shadowless edge illumination of the original while avoiding a direct reflection, for example, by the glass plate of the original holder, in the direction of the optical system. Another disadvantage in the case of the known illumination systems is that the changing of a light source is time-consuming owing to the complex structure of the illumination systems.